The present invention is directed to an ink composition containing modified pigment particles as a colorant. More specifically, the present invention is directed to an ink composition comprising a liquid vehicle and pigment particles that have been treated with a vinyl aromatic salt which has been polymerized and attached to the surfaces of the pigment particles either by chemical bonding (grafting) or by adsorption. The ink composition of the present invention is suitable for use in various applications, such as ink for writing pens, inks for conductive coatings and conductive images, and inks for printing and graphic applications. In addition, ink compositions of the present invention are particularly suitable for ink jet printing processes, including thermal ink jet printing processes, acoustic ink jet printing processes, piezoelectric ink jet and continuous ink jet printing processes.
Ink jet printing systems generally are of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. Multiple orifices or nozzles also can be used to increase imaging speed and throughput. The stream is ejected out of orifices and perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the electrically charged ink droplets are passed through an applied electrode which is controlled and switched on and off in accordance with digital data signals. Charged ink droplets are passed through a controllable electric field which adjusts the trajectory of each droplet in order to direct it to either a gutter for ink deletion and recirculation or a specific location on a recording medium to create images. The image creation is controlled by electronic signals.
In drop-on-demand systems, a droplet is ejected from an orifice directly to a position on a recording medium by pressure created by, for example, a piezoelectric device, an acoustic device, or a thermal process controlled in accordance with digital data signals. An ink droplet is not generated and ejected through the nozzles of an imaging device unless it is needed to be placed on the recording medium.
Since drop-on-demand systems require no ink recovery, charging, or deflection operations, the system is simpler than the continuous stream type. There are three types of drop-on-demand ink jet systems. One type of drop-on-demand system has an ink filled channel or passageway having a nozzle on one end and a regulated piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles necessary for high resolution printing, and physical limitations of the transducer result in low ink drop velocity. Low drop velocity may seriously diminish tolerances for drop velocity variation and directionality, thus impacting the system's ability to produce high quality copies, and also decreases printing speed. Drop-on-demand systems which use piezoelectric devices to eject the ink droplets also suffer the disadvantage of a low resolution. A second type of drop-on-demand ink jet device is known as acoustic ink printing which can be operated at high frequency and high resolution. The printing utilizes a focused acoustic beam formed with a spherical lens illuminated by a plane wave of sound created by a piezoelectric transducer. The focused acoustic beam reflected from a surface exerts a pressure on the surface of the liquid, resulting in ejection of small droplets of ink onto an imaging substrate. Aqueous based inks can be used in this system.
The third type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets and allows very close spacing of nozzles. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information generate an electric current pulse in a resistive layer (resistor) within each ink passageway near the orifice or nozzle, causing the ink in the immediate vicinity of the resistor to be heated up periodically. Momentary heating of the ink leads to its evaporation almost instantaneously with the creation of a bubble. The ink at the orifice is forced out of the orifice as a propelled droplet at high speed as the bubble expands. When the hydrodynamic motion of the ink stops after discontinuous heating followed by cooling, the subsequent ink emitting process is ready to start all over again. With the introduction of a droplet ejection system based upon thermally generated bubbles, commonly referred to as the "bubble jet" system, the drop-on-demand ink jet printers provide simpler, lower cost devices than their continuous stream counterparts, and yet have substantially the same high speed printing capability.
The operating sequence of the bubble jet system begins with a current pulse through the resistive layer in the ink filled channel, the resistive layer being in close proximity to the orifice or nozzle for that channel. Heat is transferred from the resistor to the ink. The ink becomes superheated far above its normal boiling point, and for water based ink, finally reaches the critical temperature for bubble nucleation and formation of around 280.degree. C. and above. Once nucleated and expanded, the bubble or water vapor thermally isolates the ink from the heater and no further heat can be applied to the ink. This bubble expands rapidly due to pressure increase upon heating until all the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor, which removes heat due to heat of vaporization. The expansion of the bubble forces a droplet of ink out of the nozzle located either directly above or on the side of a heater, and once the excess heat is removed with diminishing pressure, the bubble collapses on the resistor. At this point, the resistor is no longer being heated because the current pulse has been terminated and, concurrently with the bubble collapse, the droplet is propelled at a high speed in a direction towards a recording medium. Subsequently, the ink channel refills by capillary action and is ready for the next repeating thermal ink jet process. This entire bubble formation and collapse sequence occurs in about 30 microseconds. The heater can be reheated to eject ink out of the channel after 100 to 2,000 microseconds minimum dwell time and to enable the channel to be refilled with ink without causing any dynamic refilling problem. Thermal ink jet processes are well known and are described in, for example, U.S. Pat. Nos. 4,601,777, 4,251,824, 4,410,899, 4,412,224, and 4,532,530, the disclosures of each of which are totally incorporated herein by reference.
Known ink jet ink compositions generally comprise a dye which is soluble in an ink vehicle such as water or a mixture of water and a water soluble or water miscible organic solvent. The colorant in these inks is a soluble dye rather than a pigment. Inks comprising soluble dyes can exhibit many problems, such as poor waterfastness, poor lightfastness, clogging of the jetting channels as a result of solvent evaporation and changes in the dye's solubility, dye crystallization, ink bleeding when prints are formed on plain papers, poor thermal stability, chemical instability, ease of oxidation, and low drop velocity. In addition, many of the dyes contained in inks may be potentially mutagenic. These problems can be minimized by replacing some of the dyes used in ink formulations with insoluble pigments. In general, pigments have superior properties with respect to dyes, such as good waterfastness, good lightfastness, good image density, thermal stability, oxidative stability, the ability to perform intercolor ink mixing, compatibility with both coated/treated and plain papers, and nonmutagenic properties.
Ink jet inks containing pigment particles as colorants are known. For example, in Dunn, "Waterproof Carbon Black Ink for Ink Jet Printing," Xerox Disclosure Journal, Vol. 4, No. 1 (1979), a waterproof colloidal carbon black ink for ink jet printing is disclosed. The ink is prepared by incorporating a water-resistant acrylic polymer binder into an ink jet ink, such that the ink composition comprises about 9 percent by weight of carbon black, about 2 percent by weight of an anionic polymer-type dispersing agent, about 5 percent by weight of polyethylene glycol, about 8 percent by weight of Carboset 514H, and about 76 percent by weight of ammoniated distilled water. Sufficient ammonium hydroxide is added to the ink to adjust the pH to 8.5. According to the article, this ink composition is particularly suited to ink jets run in a continous mode.
In addition, U.S. Pat. No. 4,597,794 discloses an ink jet recording process which comprises forming droplets of an ink and recording on an image receiving material by using the droplets, wherein the ink is prepared by dispersing fine particles of a pigment into an aqueous dispersion medium containing a polymer having both a hydrophilic and a hydrophobic construction portion. The hydrophilic portion constitutes a polymer of monomers having mainly additively polymerizable vinyl groups, into which hydrophilic construction portions such as carboxylic acid groups, sulfonic acid groups, sulfate groups, and the like are introduced. Pigment particle size may be from several microns to several hundred microns. The ink compositions disclosed may also include additives such as surfactants, salts, resins, and dyes.
U.S. Pat. No. 3,705,043 discloses an ink suitable for jet printing which comprises a high infrared absorbing coloring component and a humectant in the form of an aliphatic polyol, alkyl ether derivatives of aliphatic polyols, and mixtures thereof in aqueous media. The infrared absorber component may be a high infrared absorptive water soluble dye, a solution of water dispersed carbon blacks, or mixtures thereof. In addition, U.S. Pat. No. 3,687,887 discloses an ink jet ink having application onto a film base which comprises an aqueous system containing 1 to 5 percent by weight of a dissolved styrene-maleic anhydride resin, 3 to 20 percent by weight of glycol ethers, and up to 4 percent by weight of carbon black in suspension or 1 to 4 percent of orthochromatic dyes in solution, or both, plus additives such as tinting dyes. Example 1 of this patent discloses a general ink formulation containing carbon black and a glycol ether, which may be an ethylene glycol type ether. Further, Japanese Patent 59-93765 discloses a recording liquid for ink jet printers. The ink disclosed therein is designed for dissolution stability at temperatures above 250.degree. C. to prevent damage to the ink jet head, and comprises a dye, a solvent such as water, an organic solvent, an optional surface tension controller, a viscosity controller, and other additives. An amount of C.I. Food Black 2 is used as the colorant, and is present in the liquid in an amount of 0.5 to 15 percent by weight.
U.S. Pat. No. 4,273,847 discloses a printing ink comprising particles of small size, each having a body portion consisting of a fusible resin with a colorant dispersed therein and an electrically conductive material, which may be carbon particles, situated substantially entirely on the surface of the body portion and comprising 5 to 10 percent of the weight of the ink. The disclosed ink is suitable for use in pulsed electrical printing.
Further, U.S. Pat. No. 4,530,961 discloses an aqueous dispersion of carbon black grafted with hydrophilic monomers of alkali or ammonium carboxylate bearing polyacrylates, which suspension may be used for manufacturing ink jet inks. The dispersion has a viscosity of about 2 to about 30 centipoise for a carbon black content of about 1 to 15 percent by weight. This composition may also contain surfactants, wetting agents, dyes, mold inhibitors, oxygen absorbers, buffering agents, pH controlling agents, and viscosity controlling agents. Carbon black particles contained in the composition are of a size that permits them to pass easily through 1 to 50 micron mesh filters. Example 9 of this reference discloses three possible ink compositions containing the carbon black suspension. In addition, at column 1, lines 43 to 46, this patent indicates that it is known to graft hydrophobic styrene onto carbon black.
The use of aromatic sulfonates as surfactants is also known. For example, U.S. Pat. No. 2,629,697, the disclosure of which is totally incorporated herein by reference, discloses a liquid composition comprising sodium alkyl aryl sulfonate, liquid hydrocarbon, and an aliphatic hydroxy compound, and optionally also containing water. The compound is useful as an emulsion type cleaning compound. In addition, U.S. Pat. No. 2,335,505, the disclosure of which is totally incorporated herein by reference, discloses a typographic printing ink vehicle which comprises alkali metal silicates dissolved in mixtures of water with water-miscible organic solvents. The ink can also contain an aromatic ester sulfonate. Further, U.S. Pat. No. 3,844,994, the disclosure of which is totally incorporated herein by reference, discloses an ink formulation useful in lithographic printing processes which is a stable emulsion comprising an oleophilic phase and a hydrophilic phase, comprising one or more polyhydric alcohols, a surfactant, and polystyrene sulfonate.
Additional references of background interest include U.S. Pat. Nos. 3,846,141, 4,165,399, 4,409,039, 4,409,040, and U.S. Pat. No. 4,508,570.
Although known inks are suitable for their intended purposes, a need continues to exist for ink compositions containing pigment particles as colorants. In addition, a need exists for ink compositions wherein pigment particles are stably dispersed in an aqueous medium. A need also exists for ink compositions suitable for ink jet printing. Further, there is a need for ink compositions that generate high quality images on a variety of substrates, including plain and treated papers, transparencies, plastic, and fabric, when employed in ink jet printing processes, including continuous ink jet printing and drop-on-demand processes. There is also a need for ink compositions suitable for use in ink jet printing systems wherein the images generated exhibit excellent image sharpness, lightfastness, waterfastness, and optical density as well as the absence of feathering and the absence of mottling in solid image areas. Further, a need exists for ink compositions that contain conductive pigment particles, wherein images generated with the liquids are electrically conductive and enable preparation of a conductive image or pattern on a nonconductive substrate. A need also exists for ink compositions that are nontoxic and nonmutagenic. There is also a need for ink compositions suitable for ink jet printing that exhibit low viscosities of less than 5 centipoise. Additionally, there is a need for ink compositions containing pigment particles with an average diameter of less than 3 microns, and preferably less than 1 micron. Further, a need remains for ink compositions capable of generating images with optical density of over 1.20 on various plain papers.